Semiconductor devices, such as an LSI, have been reduced in size, and it has become increasingly important to control distribution of an impurity introduced into a silicon substrate with high precision. For example, as for a source/drain extension of a MOS transistor, it has been conventionally performed that an impurity is ion-implanted into a silicon substrate, and the impurity thus implanted is then activated by means of activation annealing. However, since the impurity is diffused at the time of the activation annealing in this method, it is difficult to accurately control the impurity distribution.
It is known in the art that this problem can be avoided by employing such a method in which a surface layer of the silicon substrate is damaged to form an amorphous layer by ion-implanting germanium into the silicon substrate. After that, an impurity for a source/drain extension is ion-implanted into the silicon substrate so that the impurity is encompassed in this amorphous layer. According to this method, the temperature for activation annealing can be set lower compared to the case where an amorphous layer is not formed. Thus, the diffusion of the impurity due to heat can be prevented, and the impurity concentration can be easily controlled. Note that the amorphous layer is crystallized again at the time of crystallization annealing.
In the case of employing such a method, an ion implantation condition has to be determined so that a major part of an impurity for the source/drain extension would be encompassed in the range of the thickness of the amorphous layer. Hence, it is needed to obtain the thickness of the amorphous layer.
Moreover, even in the case where the germanium ion-implantation is omitted, the amorphous layer is also formed by ion-implanting the impurity for the source/drain extension. Many defects are formed in the interface between this amorphous layer and the silicon substrate which is not crystallized (that is, the bottom surface of the amorphous layer). Since the positions of the defects greatly affect characteristics of the device, it is important to obtain the thickness of the amorphous layer even in this case.
As a method for obtaining the thickness of the amorphous layer, there is a method of measuring the thickness of the amorphous layer from an image obtained by observing, with TEM (Transmission Electron Microscopy), a cross section of a sample after ion implantation, for example.
However, an ion implantation is performed many times in a semiconductor device under various implantation conditions. Thus, if observation using a TEM is performed for each ion implantation, the cost increases and a considerable amount of labor is required.
In M. Posselts, B. Schmidt, R. Groetzschel, C. S. Murthy, T. Feudel, and K. Suzuki, “Modeling of damage accumulation during ion implantation into single-crystalline silicon,” J. Electrochem. Society, vol. 144, pp. 1495-1504, 1997, a fitting parameter is provided so as to accord with experimental data in the Monte Carlo method, and thereby the thickness of an amorphous layer is quantitatively calculated. However, it is difficult to model the damage accumulation caused by ion implantation. Furthermore, a long period of time is required for calculation by the Monde Carlo method. Therefore, an ordinary device designer cannot easily use this method.
Japanese Patent Application Laid-open Publication No. 2001-230291 discloses a method of measuring the thickness of the above-mentioned amorphous layer by means of a spectroscopic ellipsometry.
Japanese Patent Application Laid-open Publication No. 2000-138178 discloses a method of calculating the lateral extension of an ion-implanted impurity.
G. Hobler, S. Selberherr, “Two-dimensional modeling of ion implantation induced point defects,” IEEE Trans. Compute-Aided Design, vol. 7, pp. 174-180, 1988 proposes an empirical model for generating defect concentration distribution from a result calculated by the Monte Carlo method.
Furthermore, the Kunihiro Suzuki, Ritsuo Sudo, Yoko Tada, Miki Tomotani, Thomas Feudel, and W. Fichtner, “Comprehensive analytical expression for dose dependent ion-implanted impurity concentration profiles,” Solid-State Electronic, vol. 42, pp. 1671-1678, 1998 shows that a vast amount of database of concentration distribution by ion implantation is present.